The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive using a polypropylene impact copolymer to provide high bond performance for making elastic components such as laminates containing elastic strands or films for use in disposable diapers.
The increasing complexity of manufactured goods, in particular disposable goods, leads to major improvements and developments in the hot melt adhesive industry. Hot melt adhesives are being used to bond a wider variety of substrates, within a broader adhesive application process window, and for a large end-use portfolio. For example, considering the diaper manufacturing industry, materials involved may be non-woven materials, polymeric films, and in general elastomeric components. These elastomeric components can be used in products like diapers, in a form of strands, films, nonwovens or any other continuous or discrete form.
Processability of hot melt adhesives are linked to their ability to be melted, and transported and/or coated in a molten stage at the final location where the bond is required. Usually the molten adhesive is sprayed, or coated as a film. Once cooled down, the adhesive needs to fulfill multiple requirements, like bond strength measured by peel force or bond retention under or after mechanical stress, and under or after various thermal conditions.
Typically hot melt adhesives can be based on polymers such as polyolefins (ethylene- or propene-based polymers), or functionalized polyolefins (ethylene or propene copolymers with oxygen containing monomers), or styrenic block copolymers containing at least one rubbery phase, like styrene-isoprene-styrene (SIS), or styrene-butadiene-styrene (SBS) polymers. Styrenic block copolymers are of interest due to their dual characteristics, i.e. cohesion of the styrenic phase associated with the rubbery behavior of another phase. Typical application temperatures are equal to or higher than 150° C.
Over the years, many different olefinic polymers have been used in the formulation of hot melt adhesives used in the construction of disposable soft goods. The first of these was amorphous polypropylene (APP). This material was produced as a by-product of crystalline polypropylene and was obtained by solvent extraction. This APP polymer could be combined with various tackifiers, plasticizers, waxes and fillers. to produce a hot melt that could be used for diaper construction, for example.
Later, olefin polymers became available that had much improved properties over the original APP polymers. These were referred to as amorphous poly alpha olefins (APAO). They were primarily produced using Ziegler-Natta catalysis and could be made using a variety of monomers, including but not limited to propylene, ethylene and butene. Various copolymers and terpolymers are produced by a number of manufacturers. They include Evonik Industries, who produce the Vestoplast® polymers; REXtac, LLC, who produces the Rextac® range of materials and Eastman Chemical, manufacturers of the Eastoflex® line of polymers. They are all characterized by having a very low degree of crystallinity as measured by DSC. As commercially produced, they are random polymers having broad molecular weight distributions.
More recently, metallocene catalysis has been used to make polyolefins with more precisely tailored properties. For example, the molecular weight of the polymer can be controlled in a way not possible with the older Ziegler-Natta catalysts. Polymers can be made using high levels of comonomer, such as butene-1 and octene-1, to produce polymers with very low levels of crystallinity and density. While these polymers have been used to make hot melt adhesives with better adhesion characteristics, they have not been widely used in conjunction with elastomeric bonding in the nonwovens industry because of their lack of adhesive bond retention. Examples of these metallocene polymers include Affinity® and Engage® polymers from Dow Chemical Company.